Motorola MPC8260 - Manual (Page 694)

25-2 MPC8260 PowerQUICC II UserÕs Manual MOTOROLA
Part IV. Communications Processor Module
The control byte within the LocalTalk frame indicates the type of frame. Control byte
values from 0x01Ð0x7F are data frames; control byte values from 0x80Ð0xFF are control
frames. Four control frames are deÞned:
¥ ENQÑEnquiry
¥ ACKÑEnquiry acknowledgment
¥ RTSÑRequest to send a data frame
¥ CTSÑClear to send a data frame
Frames are sent in groups known as dialogs, which are handled by the software. For
instance, to transfer a data frame, three frames are sent over the network. An RTS frame
(not to be confused with the RS-232 RTS pin) is sent to request the network, a CTS frame
is sent by the destination node, and the data frame is sent by the requesting node. These
three frames comprise one possible type of dialog. After a dialog begins, other nodes cannot
start sending until the dialog is complete. Frames within a dialog are sent with a maximum
interframe gap (IFG) of 200 µs. Although the LocalTalk speciÞcation does not state it, there
is also a minimum recommended IFG of 50 µs. Dialogs must be separated by a minimum
interdialog gap (IDG) of 400 µs. In general, these gaps are implemented by the software.
Depending on the protocol, collisions should be encountered only during RTS and ENQ
frames. Once frame transmission begins, it is fully sent, regardless of whether it collides
with another frame. ENQ frames are infrequent and are sent only when a node powers up
and enters the network. A higher-level protocol controls the uniqueness and transmission
of ENQ frames.
In addition to the frame Þelds, LocalTalk requires that the frame be FM0 (differential
Manchester space) encoded, which requires one level transition on every bit boundary. If
the value to be encoded is a logical zero, FM0 requires a second transition in the middle of
the bit time. The purpose of FM0 encoding is to avoid having to transmit clocking
information on a separate wire. With FM0, the clocking information is present whenever
valid data is present.
25.2 Features
The following list summarizes the features of the SCC in AppleTalk mode:
¥ Superset of the HDLC controller features
¥ FM0 encoding/decoding
¥ Programmable transmission of sync sequence
¥ Automatic postamble transmission
¥ Reception of sync sequence does not cause extra SCCE[DCC] interrupts
¥ Reception is automatically disabled while sending a frame
¥ Transmit-on-demand feature expedites frames
¥ Connects directly to an RS-422 transceiver